It is an object of the present invention to provide an improved snubber circuit for a switching transistor which significantly reduces overshoot voltage.
It is a further object of the invention to provide an improved snubber circuit which reduces overshoot voltage with minimum power lost in the snubber components.
In my present invention, a switching transistor is connected to operate at a relatively high-frequency to repetitively connect and disconnect a primary winding of a power transformer across a direct current (DC) power source. A snubber circuit is connected to absorb inductive voltages generated in the transformer winding when the transistor is turned off. The snubber circuit includes a clamp or reset winding on the transformer coupled in series with a diode across the DC power source. A junction intermediate the transistor and primary winding of the transformer is coupled via a capacitor, an inductor and a pair of series connected diodes to the junction intermediate the clamp winding and its series connected diode. An additional capacitor is connected between one terminal of the power source and a junction intermediate the pair of series connected diodes. Another terminal of the power source is connected to each end of the inductor by another pair of diodes. In this arrangement, when the transistor is turned off, the magnetizing current generated in the primary winding of the power transformer is allowed to flow into the capacitor connected to the junction between the transistor and primary winding by means of one of the diodes. The value of the capacitor is selected so as to control the rate at which charge is accumulated thereby shaping the load line of the transistor. At very high power levels when the voltage would become excessive across the transistor, the diode in series with the additional transformer winding breaks over so that further energy is returned to the DC source. Because there is considerable leakage inductance between the primary winding of the transformer and the secondary clamp winding, there may be some overshoot of the voltage before it becomes clamped to the power supply voltage. To prevent this overshoot the additional capacitor is provided so that current conduction through the clamp winding begins prior to the time at which the diode begins to conduct. The inductor and one additional diode allow the two capacitors to be reset at the end of each cycle so that further snubbing can occur at the next turnoff of the transistor.